Solid waste materials of the type typified by municipal waste traditionally have presented problems of disposal. These disposal difficulties have become increasingly critical as populations have expanded and as the per capita production of solid waste has increased. Conventionally, such solid waste has been disposed of by incineration and/or land fill. With the precent concern over problems associated with the protection of the environment, both of these traditional techniques of disposal have become undesirable. Further, separation systems, to remain efficient, must be capable of having a reasonably high throughput rate for the material processed. To the present, the throughput rates of conventional systems have not been adequately high to derive efficiencies permitting the use of such equipment in municipalities of smaller or medium size. However, because of the ever-increasing rigid requirements for carrying out waste treatment, some technique must be found to effectively increase such output rates.
Controlled composting procedures are considered to offer significant promise in the treatment of biodegradable industrial and municipal wastes and the compost product generated thereby represents a valuable and readily saleable product. A desired approach to treatment of waste by biodegradation is one where composting is carried out in the thermophilic phase at temperatures of about 45.degree.-50.degree. C. At such temperatures, bacteria otherwise active in the mesophilic phase at lower temperatures are avoided and noxious odors are not evolved in the course of digestion.
To achieve a marketable compost product, a waste treatment process should be carried out wherein raw refuse is passed through a variety of shredding or reduction, separation and related treatment stages. These stages serve to remove inorganic components such as metals and plastics and affect a reduction of average waste particle size. The segregated or separated by-product materials, such as ferrous and non-ferrous metals, glass and plastic increasingly are becoming valuable resources worthy of the expenditure of capital for effective separation equipment. Of course, the quality and resultant value of a compost by-product also is dependant upon the corresponding quality of separation, the presence of plastic or other foreign particles being undesirable or unacceptable for most commercial applications.
A broad variety of separation techniques have been known to industry, the more prominent approach being that of air separation. This technique, for example utilized in fan mills in early farming, is one wherein air is blown into the materials being separated. The density and other physical characteristics of the separated materials determines the distance which they are blown and that distance serves to effect a separation.
Where air separation techniques have been applied to municipal waste separation, designers have found that achieving high quality separation within reasonable cost limits proves to be an elusive goal. For example, municipal waste varies widely in make-up and consistency essentially from day to day. Thus, a uniform product is not available for separation treatment. Further in this regard, relatively large quantities of the material must be moved and manipulated about the treatment site and this manipulation, of itself, now has been discovered to represent a design parameter which must be accommodated for.